Method of electrically heating metallic strip



F. MlESS Jan. 19, 1943.

METHOD OF .ELECTRICALLY HEATING METALLIC STRIP Filed Dec. 20, 1941 FEEPatented Jan. 19, 1943 UNITED STATES PATENT OFFICE METHOD OFELECTRICALLY HEATING METALLIC STRIP 3 Claims.

This invention relates to a method of electrically heating metallicstrip, and more particularly to the heat treatment of strip steel,although it is not limited thereto.

In the manufacture of strip steel, it is at frequent stagesof itsmanufacture in the form of coils of substantial width and largediameter. In order to heat treat it, the strip, while in coiled form,may be placed in a furnace and subjected to heat. material to be treatedin the furnace necessitates a long period of time for the heat topenetrate to the center of the mass and for all parts to attain equalityof temperature.

The greater, the rate at which heat flows in, the greater the differencein temperature between the outside surface and the center of the mass.Therefore when the surface reaches the limiting temperature and thetemperature of the center begins to approach the temperature of thesurface, the difference between the two temperatures becomes less andthe rate of rise at the center becomes slower.

Under these conditions it may require from 50 to 75 hours (includingsoaking period) to bring a charge of coiled steel strip to a reasonablyuniform annealing temperature. Even then temperatures are not as nearlyuniform as might be desired and different parts of the mass havereceived different heat treatments due to the great differences in timethat they were held at the maximum temperatures.

With the gas-fired furnaces, heat loss through furnace walls goes on forfrom 50 to 75 hours per charge, and in addition there is an enormousloss of heat in the products of combustion, leaving the furnaceat 1300"to 1400 F. for the 50- to 75- hour run, so that the coiled steel stripbeing treated receives only about 15% of the heat of the fuel consumed.

Another method of heat treating strip steel is to pass it through anelongated furnace in continuous, as distinguished from coiled, form.This method of heat treating is not only expensive because of the highcost of the elongated furnace, but also because of the necessity ofrather elaborate handling apparatus. In addition, the construction ofthe elongated furnace is such that there are large heat radiatingsurfaces which result in the loss of heat, and the output ofheat-treated strip is slow.

It is among the objects of the present invention to electrically heattreat metallic strip in a rapid and inexpensive manner.

Another object is to provide a method of the The placing of such a largemassof' type described in which there is a large mass/surface ratio,thus permitting the treatment of a large quantity of metallic strip in afurnace of minimum dimensions, whereby heat loss is minimized.

Still another object is the provision of a method of electrically heattreating metallic strip while in coiled form when using practicablecurrents and voltages, as distingished from prior art practices whichrequire impractically large currents and low voltages.

A further'object is the provision of a method of heat treating metallicstrip wherein it is heated in such manner that all parts thereof maybe'brought to desired temperature simultaneously.

The invention, then, comprises the features hereinafter fully describedand as particularly pointed out in the claims, the following descriptionand the annexed drawing setting forth in detail certain illustrativeembodiments of the invention, these being indicative of several of thenumber of ways in which the principles of the invention may be employed.

In said drawing:

Figure 1 is a plan of an upright coil of metallic strip and illustratingdiagrammatically instrumentalities which may be used to heat it inaccordance with the teachings of the present invention.

Figure 2 is a sectional view on the line 11-11 of Figure 1.

Figure 3 is a fragmentary plan 01' the coil of Figure 1, illustratingthe manner in which insulating material is disposed between theconvolutions of the coil. I

Figure 4 is a sectional view on the line IV-IV of Figure 3.

Referring more particularly to the drawing, the numeral 2 designates afurnace or insulating cover which is provided with a top portion 3, thebottom thereof being open and adapted to rest upon a base 4.

As shown more clearly in Figure 2, the base 4 is provided adjacent oneend with a pair of adiacent but spaced vertical apertures 5 and 0 andadjacent the other end with a single aperan alternative, a continuousStrip of asbestos may be coiled with the metallic strip in such manneras to maintain the convolutions thereof in insulated relationship. Stillanother method is to coat the strip on one or both sides with a suitableinsulating paint or cement of which there are so many commerciallyobtainable varities that their specific mention herein is deemedunnecessary.

To one end of the strip I there is suitably connected, as shown at l2,an electrical tap connecting with a lead l3 which passes through theaperture 5 in the base 4 and is connected to a suitable source ofelectrical energy generally designated at H. To the other end of thestrip in there is suitably connected, as shown at H, an electrical tapto which there is connected a lead i8 which passes through the apertureI in the base 4 and is suitably connected to the source I of electricalenergy which was referred to hereinbefore. Electrical energy may besupplied to the strip through the taps l2 and I1, whereby the same maybe suitably heated by resistance. In this manner, the electric currentflows from end to end of the continuous strip with electrical insulationdisposed between the convolutions of the coil, whereby the requiredcurrent is reduced to a value which is easily handled by leads andconnectors of moderate size, and the voltage is raised to valuesobtainable from commercially acceptable electrical machinery.

It frequently happens that the strip varies in thickness, and because ofthis or any other reason. it may be deemed desirable for increasing ordecreasing the current flowing through any part of the coil. In order toprovide for this, a tap 20 is suitably connected to any desiredconvolution of the coil II), this tap 28 being connected to a lead 2|which is in turn-connected to a means 22 for increasing or decreasingthe current in that portion of the coil which is between thetap i2 andthe tap 20. This means 22 may take the form of a generator or a shunt.

In the foregoing manner, then,-current from the source 14 is passedthrough the coil ID of metallic strip in such manner that immediatelythe whole mass of coil from the exterior to the interior thereof beginsto increase in temperature at the same rate, but after a short time,however, the temperature of the exterior of the coil Ill lags behind thetemperature of the center of the mass due to radiation of heat from thesurface metal, while no such radiation takes place from the metal at thecenter. To avoid this undesirable feature, the furnace or cover 2 whichencloses the coil I0 should have its temperature raised at the same rateas the rising temperature of the coil. Thus the temperatures of the coiland enclosing furnace are balanced, and radiation from the coil isprevented, thereby permitting the coil to heat uniformly throughout bythe action of the electrical currents. The heating of the furnace orcover 2 may be accomplished in any of the manners well known in the art,and as such forms no part of the present invention, whereby theillustration of a specific heating instrumentality herein is not deemednecessary.

By placing the tap 20 within a few convolutions of the outside end ofthe coil III, the uniform heating throughout of the coil to atemperature above the maximum obtainable by the furnace is accomplished.As the electrical current raises the temperature of the coil above themaximum obtainable by the furnace, there is increased the current in theconvolutions between the taps l2 and 20 by an amount ju'st suflicient tomake up for the heat loss by radiation from the coil to. the somewhatcooler furnace, thus maintaining uniform temperatures throughout thecoil in spite of the radiation from the surface of the coil to thefurnace.

It will be readily understood by those skilled in the art that thegenerator or shunt 22 may serve either to put extra electrical currentthrough a cooler section of the coil ill, or the same effect may beobtained by shunting the hotter section of the coil.

While I have shown and described certain specific embodimentsof thepresent invention, it will be seen that I do not wish to be limitedexactly thereto, since various modifications may be made withoutdeparting from the scope of the invention as defined in the appendedclaims.

I claim:

1. The method of electrically heat treating an attenuated metallicarticle which comprises convoluting the said article, maintaining therespectively adjacent convolutions of the said article out of electricalcontact with each other, including at least a portion of the saidarticle in an electrical resistance-heating circuit, applying to saidelectrical resistance-heating circuit an electric current in sufficientvolume to raise the temperature of said article to that required for thedesired heat treatment, and imparting to the exterior convolutions ofthe said article sumcient additional heat to substantially compensatefor heat loss from the surface thereof.

2. The method of electrically heat treating an attenuated metallicarticle which comprises convoluting the said article, maintaining therespectively adjacent convolutions of the said article out of electricalcontact with each other, including at least a portion of the saidarticle in an electrical resistance-heating circuit, applying to saidelectrical resistance-heating circuit an electric current in sufficientvolume to raise the temperature of said article to that required for thedesired heat treatment, and externally applying to the exteriorconvolutions of the said article suflicient additional heat tosubstantially compensate for heat loss from the surface thereof.

3. The method of electrically heat treating an attenuated metallicarticle which comprises convoluting the said article, maintaining therespectively adjacent convolutions of the said. article out ofelectrical contact with each other, including a portion of the saidarticle in an electrical resistance-heating circuit, applying to saidelectrical resistance-heating circuit an electric current in sufficientvolume to raise the temperature of said article to that required for thedesired heat treatment, including another portion of the said article inanother electrical resistance-heating circuit, and applying to saidsecond electrical resistance-heating circuit an electric current ofgreater magnitude than that applied to said first-named electricalresistanceheating circuit.

' FRED MIESS.

